✨ (pagetop): Añade puntos de corte a Flex/FlexItem

- `Breakpoint` gana variante `Xxxl` y métodos `name()`/`min_width()`/
  `resolved()`, para consultarse contra el tema activo.
- `Theme::breakpoint_entry()` sustituye a `breakpoint_min_width()`:
  devuelve un `BreakpointEntry` con nombre y ancho mínimo.
- Nuevo `Responsive<T>`, valor en cascada mobile-first por punto de
  corte.
- Ejemplo `examples/intro-responsive.rs` con los patrones de uso.

tests/component_container.rs

@@ -143,3 +143,75 @@ async fn gap_none_adds_no_gap_style() {
     assert!(!html.contains("gap"));
     assert!(!assets.contains("gap"));
 }
+
+#[pagetop::test]
+async fn inline_flex_uses_its_own_display_value() {
+    let mut cx = Context::default();
+    let mut container = Container::new()
+        .with_flex(Flex::inline())
+        .with_child(Lc::n("x"));
+    let html = container.render(&mut cx).await.into_string();
+    let assets = cx.render_assets().into_string();
+
+    assert!(html.contains(r#"class="_inline-flex_""#));
+    assert!(assets.contains("_inline-flex_{display:inline-flex}"));
+}
+
+#[pagetop::test]
+async fn inline_flex_at_a_breakpoint_adds_its_suffix() {
+    let mut cx = Context::default();
+    let mut container = Container::new()
+        .with_flex(Flex::inline_at(Breakpoint::Lg))
+        .with_child(Lc::n("x"));
+    let html = container.render(&mut cx).await.into_string();
+    let assets = cx.render_assets().into_string();
+
+    assert!(html.contains(r#"class="_inline-flex_lg_""#));
+    assert!(assets.contains("@media(min-width:992px){._inline-flex_lg_{display:inline-flex}}"));
+}
+
+#[pagetop::test]
+async fn flex_and_flex_item_classes_keep_their_order() {
+    let mut cx = Context::default();
+    let mut container = Container::new()
+        .with_prop(PropsOp::add_classes("own"))
+        .with_flex(Flex::new().with_direction(flex::Direction::Column))
+        .with_prop(PropsOp::flex_item(
+            FlexItem::new().with_grow(flex::ItemGrow::Is1),
+        ))
+        .with_child(Lc::n("x"));
+    let html = container.render(&mut cx).await.into_string();
+
+    // The component's own classes come first, then those of `Flex`, then those of `FlexItem`,
+    // which share a single accumulator in `Props::unpack_with_flex()`.
+    assert!(html.contains(r#"class="own _flex_ _flex-direction_column_ _flex-item-grow_1_""#));
+}
+
+#[pagetop::test]
+async fn flex_without_display_adds_no_class_of_its_own() {
+    let mut cx = Context::default();
+    let mut container = Container::new()
+        .with_prop(PropsOp::add_classes("own"))
+        .with_flex(Flex::default())
+        .with_child(Lc::n("x"));
+    let html = container.render(&mut cx).await.into_string();
+
+    assert!(html.contains(r#"class="own""#));
+    assert!(cx.render_assets().into_string().is_empty());
+}
+
+#[pagetop::test]
+async fn breakpoints_add_their_suffix_and_wrap_rules_in_media_queries() {
+    let mut cx = Context::default();
+    let mut container = Container::new()
+        .with_flex(
+            Flex::at(Breakpoint::Md).with_direction_at(Breakpoint::Lg, flex::Direction::Column),
+        )
+        .with_child(Lc::n("x"));
+    let html = container.render(&mut cx).await.into_string();
+    let assets = cx.render_assets().into_string();
+
+    assert!(html.contains(r#"class="_flex_md_ _flex-direction_column_lg_""#));
+    assert!(assets.contains("@media(min-width:768px){._flex_md_{display:flex}}"));
+    assert!(assets.contains("@media(min-width:992px){._flex-direction_column_lg_"));
+}